1. Field of the Invention
The present invention relates to a silicon-based film, a forming method therefor, and a photovoltaic element.
2. Related Background Art
As a method of forming a thin silicon film showing crystalline properties, there has been employed a liquid-phase growing method such as casting method, but such method required high-temperature treatment and shows difficulty in achieving mass producibility and cost reduction.
As a method of forming a thin silicon film showing crystalline properties that is other than the casting method, Japanese Patent Application Laid-Open No. 5-136062 discloses a method of applying a hydrogen plasma processing after formation of amorphous silicon and repeating such formation and processing to obtain a polycrystalline silicon film.
In a photovoltaic element employing a thin silicon film showing crystalline properties, it is generally known that the mobility of carriers is hindered for example by the influence of dangling bonds of silicon at the crystal boundary, strain generated in the vicinity of the crystal boundary, imperfection in the crystal itself etc., thereby adversely influencing the photoelectric characteristics of the photovoltaic element.
In order to alleviate the above-mentioned influences, it is considered effective to improve the crystallinity and crystalline properties, and to increase the crystalline grain size thereby reducing the density of the crystal boundaries. For achieving the above, there has been tried to reduce the film forming rate or to execute film formation by repeating formation of a silicon film and annealing the film in a hydrogen atmosphere, but such processes have been a factor of extending the film forming time thereby raising the cost.
In consideration of the foregoing, an object of the present invention is to provide a method for forming a silicon-based thin film, which makes it possible to form a silicon thin film of excellent photoelectric characteristics with a film forming rate of an industrially practical level, thereby overcoming the above-described problems; and a photovoltaic element of excellent characteristics, adhesion and environment resistance employing thus formed silicon-based thin film.
According to the present invention, there is provided a method of forming a silicon-based film by plasma CVD, which comprises:
introducing a source gas containing silicon halide and hydrogen into the interior of a vacuum vessel, at least a part of the interior being covered with a silicon-containing solid; and
generating plasma in the space of the interior of the vacuum vessel, and
forming a silicon-based film on a substrate provided in the interior of the vacuum vessel.
According to the present invention, there is also provided a silicon-based film formed by the aforementioned forming method.
According to the present invention, there is also provided a photovoltaic element comprising semiconductor layers constituting at least a set of pin junction on a substrate, wherein: at least an i-type semiconductor layer is a silicon-based film formed by the aforementioned forming method.
The silicon-containing solid is a coating film formed in a region surrounding the plasma generating space in the vacuum vessel, and preferably has a resistivity higher than that of the vacuum vessel.
The coating film is preferably formed by a method which comprises:
introducing gases containing a silicon-containing gas into the interior of the vacuum vessel prior to the introduction of the source gas;
generating plasma in the space in the vacuum vessel; and
forming a coating film by plasma CVD.
The silicon-based film preferably contains microcrystals of a crystal grain size having Scherrer radius of 10 nm or more.
The film forming rate of the silicon-based thin film after formation of a coating film having a resistivity higher than that of the vacuum vessel is preferably three or more times the film forming rate of the silicon-based thin film without formation of such coating film of higher resistivity.
The film forming rate of the silicon-based thin film after formation of a coating film having a resistivity higher than that of the vacuum vessel is preferably 1.0 nm/sec or higher.
The coating film preferably has a resistivity higher than that of the silicon-based film.
The coating film having a resistivity higher than that of the silicon-based film is preferably composed of silicon.
The gases containing the silicon-containing gas as a main component are preferably free from silicon halide gas.
The silicon halide preferably contains at least one kind of fluorine atom and chlorine atom.
The Raman scattering intensity resulting from the crystalline component of the silicon-based film is preferably three or more times the Raman scattering intensity resulting from the amorphous component.
In the X-ray or electron beam diffraction of the silicon-based film, the proportion of the diffraction intensity of (220) is preferably 50% or more of the total diffraction intensity.
The internal pressure of the vacuum vessel at the formation of the silicon-based film is preferably 50 mTorr (6.7 Pa) or higher.
Also in consideration of the above-mentioned problems, another object of the present invention is to provide a method of forming a silicon thin film showing excellent photoelectric characteristics and excellent uniformity even in a large area with a film forming rate of industrially practical level, and a photovoltaic element employing the silicon-based film formed by such method.
In the present invention, the aforementioned silicon-containing solid is preferably a member having at least a surface composed of silicon.
The above-mentioned member is preferably at least one selected from a plate-shaped silicon, a silicon grain, and a solid having a silicon film coating at least a part of the surface thereof.
The aforementioned substrate is preferably in a positional relationship opposed to a high frequency introducing portion provided in the vacuum vessel.
The aforementioned member is preferably placed on the aforementioned high frequency introducing portion.
The aforementioned silicon halide preferably contains at least one kind of a fluorine atom and a chlorine atom.
The aforementioned silicon-based film preferably includes microcrystals of a crystal grain size having a Scherrer radius of 20 nm or more.
The Raman scattering intensity resulting from the crystalline component of the silicon-based film is preferably three or more times the Raman scattering intensity resulting from the amorphous component.
In the X-ray or electron beam diffraction of the silicon-based film, the proportion of the diffraction intensity of (220) is preferably 70% or more of the total diffraction intensity.
The aforementioned silicon-containing solid is preferably a member having at least a surface composed of silicon.